1. Field of the Invention
The invention relates to a vehicle seat and specifically to a vehicle seat including a headrest with a pad.
2. Description of Related Art
A typical seat back of a vehicle seat provided in a vehicle such as an automobile has a headrest attached thereto so as to be capable of receiving the head of an occupant. Such a headrest has a cushioned pad. Some pads have an insert member at a portion at which the head of an occupant is received. As shown in FIG. 5, Japanese Patent Application Publication No. 2006-69286 (JP 2006-69286 A) describes a headrest 104 in which an insert material 146 is made of a low-resilience material. With this configuration, the sinking of the front surface portion of the headrest 104 and the bouncing of the head of an occupant (both not shown) may be reduced when a rear-side collision occurs in an automobile. Therefore, the protection of the occupant may be improved.
In the case of a rear-side collision, however, an occupant is forced to move forward by the impact of the rear-side collision. At this time, the head of the occupant is away from the headrest 104. Therefore, acceleration generated when the occupant is forced to move forward is applied to the body of the occupant prior to the head of the occupant due to an inertia force applied to the occupant (see FIG. 6). As a result, the occupant is forced to tilt the head backward (i.e., a state in which the chin of the occupant is lifted that will be hereinafter referred to as a “backward-tilted state”) toward a state in which the head of the occupant is bent backward. Then, the head put in the backward-tilted state is forced to come in contact with the headrest 104 and starts to sink into the headrest while being put in the backward-tilted state due to the inertia force continuously applied to the occupant. According to the headrest described in JP 2006-69286 A, when the head is forced to sink into the headrest in this way, a difference in the applied acceleration between the head and the body of the occupant (A1 in FIG. 6) remains great when a certain period of time elapses after the rear-side collision (time T in FIG. 6, for example). Therefore, a neck injury criterion (NIC) value calculated from the difference in the acceleration also remains great. This may negatively affect the body of the occupant.
In addition, when the head is forced to further sink into the headrest while being put in the backward-tilted state due to the inertia force continuously applied to the occupant, the rigidity of the low-resilience material also increases correspondingly and the head becomes difficult to sink into the headrest. If the sinking of the head is insufficient, the backward-tilted state may be maintained and cause negative affect on the body of the occupant.